The purpose of the proposed research is to quantify the mechanical properties of cardiac mitral valve tissue and their relationship to the microstructure. Since in vivo stress cannot be measured directly in the valve a combination of experimental data and engineering analysis is needed to estimate the distribution of different stress components. One essential component of a stress prediction is a constitutive relation, formally describing the relationship between tissue stress and strain. This application proposes to formulate a suitable constitutive relation for mitral valve tissue, incorporating regional variations and a relationship to the underlying tissue microstructure. Efforts to understand the normal stress distribution in the valve and its microstructural basis may yield insight into the role of stress in structural remodeling in disease states